1. Field of the Invention
The present invention relates to devices used in orthognatic surgery interventions or in the preparation thereof. Such interventions are surgery interventions of repair, in particular, of a mispositioning of the jaws with respect to each other. An orthognatic surgery intervention especially consists of performing osteotomies of the maxilla and/or of the mandible to reposition them correctly with respect to the rest of the skull by recreating a defective bite.
2. Discussion of the Related Art
The preparation of such a surgery intervention requires implementing orthodontic and radiological techniques.
A mandibular casting and a maxillary casting providing the respective implantations of the patient's teeth in the respectively mandibular and maxillary bone segments are first performed. The castings, generally made of plaster, are used to simulate the relative displacement which has to be applied to the jaws to recreate the bite. To enable the surgeon to respect these simulated relative positions, a plate comprising, on each of its surfaces, tooth-prints of the two castings is made with the dental castings. Such a plate, called an interscupidation plate, is used to maintain the castings or the jaws in relative positions where the teeth are in occlusion.
Since the surgical intervention generally includes osteotomies of both jaws, two interscupidation plates are generally made from the dental castings, in addition to a so-called initial interscupidation plate linking the two jaws in their occlusion position before the intervention.
A so-called intermediary plate determines the foreseeable displacement of the maxilla with respect to the mandible when said mandible is in its original (preoperative) position. This plate enables the surgeon to place the maxilla back on the skull in the desired definitive position before intervening on the mandible. A so-called definitive plate determines the occlusion objective to be surgically achieved and is thus used to correctly position the mandible on the skull by setting the position of the mandible with respect to the previously replaced maxilla.
The preparation of the surgical operation also uses a profile radiography of the patient enabling, in particular, performing an approximate simulation of the operative action.
This simulation is performed manually from a tracing paper placed on the radiography. For example, the contours of the mandible are first drawn. The tracing paper is then moved to approximately reproduce thereon the desired postoperatory occlusion, after which the maxillary contours are drawn. The maxillomandibulary assembly drawn on the tracing paper is then moved in one block while respecting cephalometric standards, labial ratios, as well as other criteria known for this type of intervention. The direction and amplitude of the jaw displacements are thus radiologically and approximately defined. The results of this simulation are compared and adjusted according to the relative motion of the mandible and of the maxilla envisaged by means of the interscupidation plates.
The actual simulation of an orthognatic surgery intervention is thus performed essentially manually. Further, this simulation is only done in two dimensions based on a plane profile view of the skull.
The development of scanners associated with image processing systems enables obtaining three-dimensional views of a patient's skull. Such systems would be particularly useful to perform a three-dimensional simulation of an orthognatic surgery intervention. In particular, it is known to isolate from one another different portions of the three-dimensional images reconstructed from scanner cross-sections. Thus, a portion corresponding to the maxilla and a portion corresponding to the mandible could be isolated from the rest of the skull. This would enable simulating, by means of the image processing system, relative displacements of these elements with respect to one another. However, the sole use of such three-dimensional image processing systems for a simulation of an orthognatic surgery intervention remains up to now impossible for several reasons.
First, the accuracy of a scanner is incompatible with the accuracy requirement of a bite. Indeed, the jaw positioning accuracy required for the bite is on the order of one tenth of a millimeter while the minimum pitch between two scanner tomographies ranges from approximately two to five millimeters. The respective initial positions of the mandible and of the maxillary thus cannot be precisely reproduced by means of the scanner.
Second, teeth amalgams (fillings) create artifacts which appear as blurred spots on the scanner images. It is thus impossible to plot, on a three-dimensional view, the exact position of the teeth based on the scanner images to obtain the bite.
A technique of preparation and assistance of an operatory action in orthognatic surgery using a scanner is however known. This technique consists of affixing three titanium screws on the patient's maxillary. A resin model of the skull is then made based on scanner cross-sections of the patient's skull. Since the screws appear on the scanner views, they are reproduced on the model. The screws are used to position with respect to the skull a metal frame for receiving a final interscupidation plate. This plate is made based on maxillary and mandibulary dental castings taken on the patient. Once the castings are made, the maxillary is cut-off from the resin model to be replaced with the corresponding casting. The maxillary casting is attached to the model in the desired definitive position. Then, the mandible is cut-off from the model to be replaced with a casting previously made on the patient.
The position of the mandibular casting is given, with respect to the maxillary, by the interscupidation plate which is then rigidly coupled to the metal frame forming a system of transfer of the plate position between the model and the patient. The frame is then brought back on the patient in the position defined by the three maxillary screws and is attached to the patient's skull by two additional screws. The position of the transfer system being now fixed by these two screws, the osteotomy of the maxillary, which is correctly repositioned by means of the interscupidation plate which is rigidly coupled to the transfer system, is performed. Then, the osteotomy of the mandible is performed, and said mandible is correctly positioned by means of the interscupidation plate.
Such a technique has several drawbacks. On the one hand, it requires an additional surgical intervention to place the screws in the patient's mouth. On the other hand, it requires making a resin model of the skull, which is particularly costly. Further, the simulation is performed, empirically, by means of the resin model and does not enable taking account of cephalometric standards based on accurate data.